How do I get an IL bytearray from a DynamicMethod?

Question

As a bit of a novelty, I'm trying to see how different the IL from light weight code generated at runtime looks vs code generated by the VS compiler, as I noticed that VS code tends to run with a different performance profile for things like casts.

So I wrote the following code::

Func<object,string> vs = x=>(string)x;
Expression<Func<object,string>> exp = x=>(string)x;
var compiled = exp.Compile(); 
Array.ForEach(vs.Method.GetMethodBody().GetILAsByteArray(),Console.WriteLine);
Array.ForEach(compiled.Method.GetMethodBody().GetILAsByteArray(),Console.WriteLine);

Unfortunately, this throws an exception as GetMethodBody is apparently an illegal operation on code generated by expression trees. How can I in a library manner (i.e. not with an external tool unless the tool has an API) look at the code generated by code using lightweight codegen?

Edit: the error occurs on line 5, compiled.Method.GetMethodBody() throws the exception.

Edit2: Does anyone know how to recover the local variables declared in the method? Or is there no way to GetVariables?

Answer 1

Yeah, doesn't work, the method is generated by Reflection.Emit. The IL is stored in the MethodBuilder's ILGenerator. You can dig it out but you have to be pretty desperate. Reflection is needed to get to the internal and private members. This worked on .NET 3.5SP1:

using System.Linq.Expressions;
using System.Reflection;
using System.Reflection.Emit;
...

        var mtype = compiled.Method.GetType();
        var fiOwner = mtype.GetField("m_owner", BindingFlags.Instance | BindingFlags.NonPublic);
        var dynMethod = fiOwner.GetValue(compiled.Method) as DynamicMethod;
        var ilgen = dynMethod.GetILGenerator();
        var fiBytes = ilgen.GetType().GetField("m_ILStream", BindingFlags.Instance | BindingFlags.NonPublic);
        var fiLength = ilgen.GetType().GetField("m_length", BindingFlags.Instance | BindingFlags.NonPublic);
        byte[] il = fiBytes.GetValue(ilgen) as byte[];
        int cnt = (int)fiLength.GetValue(ilgen);
        // Dump <cnt> bytes from <il>
        //...

On .NET 4.0 you'll have to use ilgen.GetType().BaseType.GetField(...) because the IL generator was changed, DynamicILGenerator, derived from ILGenerator.

Answer 2

The ILReader here should work.

ILVisualizer 2010 Solution

Answer 3

The current solutions here aren't addressing the current situation in .NET 4 very well. You can use either DynamicILInfo or ILGenerator to create the dynamic method, but the solutions listed here do not work with DynamicILInfo dynamic methods at all.

Whether you use the DynamicILInfo method of generating IL or the ILGenerator method, the IL bytecode ends up in DynamicMethod.m_resolver.m_code. You don't have to check both methods and it's a less complex solution.

This is the version you should be using:

public static byte[] GetILBytes(DynamicMethod dynamicMethod)
{
    var resolver = typeof(DynamicMethod).GetField("m_resolver", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(dynamicMethod);
    if (resolver == null) throw new ArgumentException("The dynamic method's IL has not been finalized.");
    return (byte[])resolver.GetType().GetField("m_code", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(resolver);
}

See this answer for more helper methods and a solution for the DynamicMethod token resolution issue.

Answer 4

Based off Hans Passant's work I was able to dig a little deeper there appears to be a method that you should call, called BakeByteArray so the following works::

var dynMethod = fiOwner.GetValue(compiled.Method) as DynamicMethod;
var ilgen =dynamicMethod.GetILGenerator();
byte[] il = ilgen.GetType().GetMethod("BakeByteArray", BindingFlags.NonPublic | BindingFlags.Instance).Invoke(ilgen, null) as byte[];

This certainly helps, but I still have no way to resolve VariableInfo's just yet which is something that would help in my work.

Answer 5

I just merged @Hans Passant and @jnm2 solutions into a extension method and added useful comments:

public static byte[] GetIlAsByteArray(this DynamicMethod dynMethod)
{

    // First we try to retrieve the value of "m_resolver" field,
    // which will always be null unless the dynamic method is completed
    // by either calling 'dynMethod.CreateDelegate()' or 'dynMethod.Invoke()' function.
    // Source: https://learn.microsoft.com/en-us/dotnet/api/system.reflection.emit.dynamicmethod.getilgenerator
    // (Remarks section)

    // Note that the dynamic method object does not know when it is ready for use
    // since there is not API which indicates that IL generation has completed.
    // Source: https://referencesource.microsoft.com/#mscorlib/system/reflection/emit/dynamicmethod.cs,7fc135a2ceea0854,references
    // (Comment lines)

    // So, if the dynamic method is not completed, we will retrieve the "m_ILStream" field instead.
    // The only difference I notice between "m_resolver" and "m_ILStream" fields is that the IL bytes in "m_ILStream"
    // will have trailing zeros / null bytes depending on the amount of unused bytes in this buffer.
    // ( The buffer size for "m_ILStream" is allocated by a call to 'dynMethod.GetILGenerator(streamSize)' function. )

    BindingFlags bindingFlags = bindingFlags.Instance | bindingFlags.NonPublic;

    object fiResolver = typeof(DynamicMethod).GetField("m_resolver", bindingFlags).GetValue(dynMethod);
    if (fiResolver == null)
    {

        ILGenerator ilGen = dynMethod.GetILGenerator();
        FieldInfo fiIlStream = null;

        // Conditional for .NET 4.x because DynamicILGenerator class derived from ILGenerator.
        // Source: https://stackoverflow.com/a/4147132/1248295
        if (Environment.Version.Major >= 4)
        {
            fiIlStream = ilGen.GetType().BaseType.GetField("m_ILStream", bindingFlags);
        }
        else // This worked on .NET 3.5
        {
            fiIlStream = ilGen.GetType().GetField("m_ILStream", bindingFlags);
        }
        return fiIlStream.GetValue(ilGen) as byte[];

    }
    else
    {
        return (byte[])(fiResolver.GetType().GetField("m_code", bindingFlags).GetValue(fiResolver));

    }

}